Mineral-magnetic proxies of erosion/oxidation cycles in tropical maar-lake sediments (Lake Tritrivakely, Madagascar): paleoenvironmental implications

Mineral-magnetic measurements were performed on the upper 16 m of five piston-cored sedimentary sequences from Lake Tritrivakely, Madagascar. AMS 14C dating and correlation of the five magnetic susceptibility records allowed establishment of a composite sedimentary record of the last 46 kyr. Our data suggest that mineral-magnetic changes result from changes in concentration of strongly ferrimagnetic terrigenous minerals and from preservation/dissolution cycles of detrital iron oxides (primarily high-coercivity Fe(III) phases such as hematite) in alternately oxic and anoxic depositional environments. High-coercivity iron oxide phases are restricted to stratigraphic intervals that are interpreted to represent dry periods with oxic conditions. Comparison with other paleoenvironmental proxies (pollen, total organic carbon, lithology) suggests that the sediments record relatively humid conditions before 22 kyr BP, and relatively dry conditions during the Last Glacial Maximum (LGM) and the early Holocene. Such results contrast with the previous observations of relatively dry glacial conditions and humid mid-Holocene conditions in the summer rainfall area of East and SE Africa.